What is thermal comfort?
Thermal comfort is affected by two types of parameters.
Such as air temperature (alternatively operative air temperature), vertical air temperature difference, mean radiant temperature, radiant temperature asymmetry (otherwise floor surface temperature), relative humidity and air speed (velocity including draughts and ventilation), and
Such as metabolic rate and clothing insulation.
Effects of air movement on people - how draught influnces comfort
|Air velocity||Probable impact|
|Up to 0.25 m/s||Unnoticed|
|0.25 to 0.5 m/s||Pleasant|
|0.5 to 1 m/s||Generally pleasant, but causes a constant awareness of air movement|
|1 to 1.5 m/s||From slightly drafty to annoyingly drafty|
|Above 1.5 m/s||Requires corrective measures if work and health are to be kept at comfortable levels|
Adapted versus not-adapted?
There are quite big differences between the European recommendations and those listed by ASHRAE. One major reason is that ASHRAE requirements are minimum code requirements, where the basis for design is adapted people, while the European recommendations are for un-adapted people (visitors). Adapted people are considered those who entered a room and are already acclimatized to the situation in the room, i.e. approximately 15 minutes after entering a room. Visitors are people who just entered a room and are assessing the situation in the room at that moment of entering.
[* Picture shows the required ventilation rates from standard EN15251 compared to ASHRAE 62.1.]
Who should we ventilate for? For people just entering the room (un-adapted) or for people already occupying a room (adapted)? Here the philosophy adopted by ASHRAE 62.1 and EN15251 differs. But should it really be one or the other? In a conference room, auditorium or lecture room most people enter at the same time. It then takes some time before the odour level has reached an unacceptable level and meanwhile people adapt. In this case it may be appropriate to require a ventilation rate based on adapted persons. There may be other spaces where you would design for un-adapted people, e.g. in a first class restaurant, offices, and department stores. It seems logical that more differentiated criteria could be used.
[*source Bjarne Olesen https://www.rehva.eu/fileadmin/hvac-dictio/01-2011/Standards_for_Ventilation_and_Indoor_Air_Quality_in_relation_to_the_EPBD.pdf]